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In petroleum drilling technology, a 'drill string' in an oil rig is the column, or string, of drill pipe with attached tool joints that transmits fluid and rotational power from the kelly drive or top drive to the drill collars and bit. Often, especially in the oil patch, the term is loosely applied to both drill pipe and drill collars. Some type of drilling fluid is almost always pumped down the inside of the drill string and circulated back up the annulus, or ring shaped void between the drill string and the formation.

Contents

Drill string components

BHA

Transition pipe

Drill pipe

Running a drill string

Stuck drill string

Drill string components

The drill string is typically made up of 3 components:

★ Bottomhole assembly (BHA)

★ Transition pipe (Often HWDP - Heavy Weight Drill Pipe)

★ Drill Pipe
Each component is made up of several parts. The various tools in each component are joined together using special threaded connections known as tool joints.

BHA

The BHA is made up of a drill bit which is used to drill the hole, ''drill collars'' which are large OD tubulars used to apply weight to the drill bit, and ''stabilizers'' which keep the drilling assembly centered in the hole. The BHA may also contain other components such as a downhole motor, Rotary Steerable System, measurement while drilling (MWD), and logging while drilling (LWD) tools.

Transition pipe

Heavyweight drill pipe (HWDP) is used to make the transition between the drill collars and drill pipe. The function of the HWDP is to provide a flexible transition between the drill collars and the drill pipe. This helps to reduce the number of fatigue failures seen directly above the BHA. A secondary use of HWDP is to add additional weight to the drill bit.

Drill pipe

Drill pipe makes up the majority of a drill string. A drill string is typically about 15,000 feet in length for an oil or gas well vertically drilled onshore in the United States and may extend to over 30,000 feet for an offshore deviated well.

Running a drill string

Most components in a drill string are manufactured in 31 foot lengths (range 2) although they can also be manufactured in 45 foot lengths (range 3). Each 31 foot component is referred to as a joint. Typically 2 or 3 joints are joined together to make a stand.
Pulling the drill string out of or running the drill string into the hole is referred to as 'tripping'. Drill pipe, HWDP and collars are typically tripped in stands to save time.

Stuck drill string

A stuck drill string is generally caused by the drill pipe sticking to the mud cake on the wall of the wellbore due to filtrate loss in wall of the well or due to cuttings settling back into the wellbore when drill fluid circulation is stopped. Stuck pipe is also possible when hydrostatic head is lower than necessary. Differentially stuck pipe occurs when a vacuum is formed between the drill bore and the drill string. Keyhole sticking occurs mechanically as a result of pipe tripping into doglegs. The BHA will also become stuck when the drillers slides without rotating, the BHA will also be packoff if not reamed out after every stands...